The invention relates generally to telemetry systems and more particularly to channel assignments associated with telemetry systems. More particularly, still, the invention relates to channel frequency assignments for multichannel telemetry systems for the purpose of reducing cross talk.
Telemetry systems basically comprise means for transmitting information from one location to a "remote" receiving location. The transmission is via radio waves rather than through a direct physical connection and it is with the former type of telemetry system that the present invention is principally involved.
Telemetry systems may comprise only one or a large number of transmit-receive channels. In the event two or more transmit-receive channels are used, it is possible for spurious electrical signals appearing in the transmitted frequency spectrum at a frequency closely adjacent that of another receiver fundamental channel frequency of the telemetry system to interfere with and/or override the information normally conveyed by the proper transmitter. The present invention is particularly concerned with telemetry systems having a relatively large number of transmit-receive channels and in which the problem of cross talk is created by spurious signals originating with the various other channels of the telemetry system.
The spurious signals from other telemetry channels which may cause the problem of cross talk can arise from the manner in which the channel or carrier frequency for the respective channels is developed. For example, it is common to generate the carrier frequency for a particular channel by multiplying a respective basic frequency obtained from a crystal oscillator or the like. In the process of frequency multiplication, various harmonics of the basic oscillator frequency are produced and these harmonics may be of such frequency and strength to interfere with fundamental frequencies of other telemetry channels.
In a significant exemplary situation, multi-channel telemetry systems are finding increasing use in hospitals and similar health-care facilities for the purpose of transmitting physiological information from a patient to a remote receiver. In such systems, the patient is normally ambulatory and may move about the hospital. The various receivers for the respective channels may all be connected at a central location to an antenna system installed within the hospital. The receiver associated with each telemetry channel is appropriately identified and may be extended to a display or recording means for visual and/or written read-out of the particular information. However, because of the aforementioned problem of cross talk, it is possible and not uncommon for the information signal from a patient on a particular telemetry channel to be obliterated or replaced by a spurious signal associated with a different patient and telemetry channel. This situation may arise where the patient on the channel being monitored moves relatively distant from the antenna system and a patient wearing a transmitter having spurious signals approximating the fundamental frequency of the channel being monitored moves relatively close to the antenna system such that the spurious signal becomes dominant.
It will be appreciated that distortion of information being received from a particular patient may be particularly undesirable and more importantly, substitution of another patient's physiological information therefor can create a potentially dangerous situation. Although certain filtering techniques and equipment may substantially reduce this problem, their relatively high cost comprises a significant deterrent to their use. In order to avoid the interference and/or confusion which may result from cross talk, efforts have been made to assign the frequencies of the respective channels such that the cross talk is either minimized or eliminated.
The usual practice has been to identify a series of tentative channel frequencies spaced from one another at random or arithmetically spaced intervals. This requires individual inspection of each tentative channel to determine the absence of cross talk from spurious signals generated by the harmonics of the respective basic frequencies of the respective oscillators. Such techniques have been extremely time-consuming and have generally not made optimum use of the available operating band.
Accordingly, it is a principal object of the present invention to provide an improved multi-channel telemetry system and a method of manufacture thereof which, in a relatively economic manner, optimizes the assignment and utilization of channel frequencies within a band with a minimum of cross talk.
This and other objects will be in part obvious and in part pointed out in greater detail hereinafter.